1. Field of the Invention
The present invention relates to an arrangement and method for utilizating waste heat comprising a waste heat exchanger, at least two turbines, at least two regenerators, at least two cooler units, and at least two pumps and/or compressors as components, where the waste heat exchanger heats up a fluid with heat from a waste heat source, and the heated fluid flows through a first set of the at least one turbine, the at least one recuperator, the at least one cooler unit, and the at least one pump and/or compressor.
2. Description of the Related Art
Organic Rankine Cycles (ORC) are used to utilize waste heat, for example, from power generation, technological processes in metal manufacturing, glass production, chemical industry, from compressors, or internal combustion engines. Conventional ORC technology is only able to use a certain amount of waste heat due to the limited thermal stability of organic fluids. It limits the thermal efficiency of ORC systems if heat source temperature exceeds 250 to 300° C. On average, the total efficiency of conventional ORC units do not exceed values of 10%. 90% of thermal energy is wasted to the atmosphere.
In very compact systems, the use of Supercritical CO2 (S—CO2) cycles allows the utilization or waste heat with an efficiency of up to 20%. The size of the system is half of that using standard ORC technology, which permits its use to utilize waste heat from different heat sources.
Substantially there are two basic conventional system layouts for S—CO2 cycles, i.e., regenerative and non-regenerative. The two cycle systems differ from each other by the presence or absence of intermediate heating of cycle fluid by expanded fluid downstream the turbine in recuperators. Both system layouts are used to utilize heat from sources with low power and temperature level with help of ORC and S—CO2 cycles.
The internal thermal efficiency of regenerative cycles is almost twice as high as the efficiency of non-regenerative cycles. It can exceed 30% for S—CO2 cycle systems. However, in real conditions of S—CO2 cycle implementation net efficiency, the rate of total available thermal to electrical energy conversion, for systems with simple layouts is around 10% of available thermal energy in the range from environmental to heat source temperature. To improve the performance and achieve 20% efficiency, more complex system layouts have to be used.
S—CO2 cycle implementation, depending on the environmental conditions and layout, may require both pumps for liquefied CO2 flow and compressors for S—CO2 gas compression. At real conditions, regenerative cycles have more than twice higher internal thermal efficiency than non-regenerative cycles and draw less thermal energy from the heat source. Even for relatively low temperatures of heat sources, temperatures at the heater outlet in regenerative cycles remain relatively high, which allows utilization of remaining thermal energy in sequentially located units.
To improve S—CO2 system efficiency, a simple sequential arrangement of at least two independent S—CO2 systems is possible, in series one after another within a gas flow with waste heat. In the sequential arrangement, the second S—CO2 regenerative cycle utilizes the heat downstream to the first regenerative cycle providing noticeable overall higher net efficiency of the waste heat utilization arrangement.
WO2012074905A2 and WO2012074911A2, for example, disclose more complex sequential arrangements of two S—CO2. The two sequentially arranged regenerative S—CO2 systems in a heat utilizing unit, described in these conventional arrangements comprise in both cases one common/merged cooler. Here, the advantage is a reduction of components, because only one cooler is required. The system complexity rises and the control gets more complicated because mass flow must be internally distributed between two turbines and united in a single cooler. In WO2012074905A2, pumps are used, assuming liquefied CO2 subsequently flows to the cooler. In WO2012074911A2, compressors are used, assuming a supercritical CO2 gas subsequently flows to the cooler.
A further integration is achieved upon joining the heaters into a single unit, as described, for example, in WO2011119650A2 and WO2012074940A3. Both layouts of regenerative S—CO2 systems comprise two expansion turbines, two regenerators but only one joint heater, one joint cooler and one pump for liquid CO2 flow. Here, less components than in the previously-described systems are provided, but they require more complex flow management. Two flow streams are joined at one point of the system and re-split to separate streams at another point of the system at an upstream location.
In WO2011119650A2, the flow stream is split up after a pump, and one flow portion is directly forwarded to a waste heat exchanger. In WO2012074940A3, before the split, the flow passes through a regenerator placed downstream the pump and only after that does the flow portion enter the waste heat exchanger.
The above-described different layouts of S—CO2 system arrangements differ in thermodynamic processes, exhibit different efficiencies, comprise different hardware components, and demand different system mass flow management and control, requirements. A reduction of components requires an increased effort for mass flow-management and control. Savings from components lead to increased costs for control and higher complexity with potentially increased error rates.